The invention relates to a method for automatically balancing the volumetric filling of cavities, in particular of multiple injection molds, and to a corresponding multiple injection mold.
For cost reasons, it is often the case that a plurality of parts are produced simultaneously each injection cycle when producing all kinds of injection moldings, for example from plastic, metal, ceramic, etc. To do this, multi-impression molds are used, the individual cavities normally being balanced out in terms of geometry and gates to a sufficient extent for the filling to be as uniform as possible.
In practice, however, fluctuations in material and temperature and resultant fluctuations in viscosity of the injection compound mean that the filling of the individual cavities always differs and also changes constantly. To achieve a molding quality which is as constant as possible, therefore, the volumetric filling of the individual cavities has to be permanently monitored and, if necessary, adjusted. In principle, the volumetric filling can be adjusted by changing the individual hot runner temperatures of the individual cavities.
U.S. Pat. No. 6,090,318 describes a method for controlling the hot runner heating in a multiple-cavity injection mold, in which the profiles of the mold internal pressure in all the cavities is recorded and the times from the start of the filling operation until a suitably selected reference pressure are measured for each cavity. By intervening in the individual heating means of the individual hot runners, the temperature in these runners are altered in such a way that the measured times for the individual cavities move closer together.
A fundamental drawback of this method is the high cost of the system for the sensors and amplifiers. Furthermore, the temporal pressure profile of the individual pressure curves is compared with mean value curves, requiring detailed process knowledge and making universal deployment more difficult.
It is an object of the present invention to significantly simplifying the above-mentioned method and achieve significant cost savings.